1. Field of Invention
The present invention relates to a switching regulator and a control method for controlling a switching regulator; particularly, it relates to a switching regulator and a control method for controlling a switching regulator which gradually decreases the output voltage to a shutdown level according to the input voltage when the switching regulator enters a shutdown procedure.
2. Description of Related Art
FIG. 1A shows a conventional power supply circuit. As shown in FIG. 1A, the power supply circuit 1 comprises a filter circuit 12 and two synchronous buck switching regulators 14 and 16, respectively filtering and converting the input voltage Vin (for example, 12V) to the output voltages Vo1 (for example, 12V), Vo2 (for example, 5V) and Vo3 (for example, 3.3V). When the power supply circuit 1 enters a shutdown procedure, the signal waveforms of the input voltage Vin and the output voltages Vo1, Vo2 and Vo3 are as shown in FIG. 1B. The so-called shutdown procedure means that the input voltage Vin begins to decrease, and when it is below an Under-Voltage Lockout threshold UVLOth, the synchronous buck switching regulators 14 and 16 stop converting the input voltage Vin to the output voltages Vo2 and Vo3, and the load circuits start decreasing the output voltages Vo2 and Vo3 until they drop to a shutdown threshold (typically 0V).
Please refer to FIG. 1B in conjunction with FIG. 1A. When the shutdown procedure begins at the time point t0, the input voltage Vin begins to decrease. When the input voltage Vin decreases to the Under-Voltage Lockout threshold UVLOth at the time point t1, the synchronous buck switching regulators 14 and 16 stop regulating the output voltages Vo2 and Vo3. At this moment, if the load circuit receiving the output voltages Vo3 (3.3V) is in a light load or no load condition, while the load circuit receiving the output voltages Vo2 (5V) is in a heavy load condition, then the loading current I2 is greater than the loading current I3. In this case, the charges stored in the output capacitor Co3 will be discharged by a slower speed than the charges stored in the output capacitor Co2. As a result, during the shutdown procedure, an undesired situation occurs (indicated by the arrow shown in FIG. 1B) where the output voltage Vo3 (3.3V in a normal operation) is greater than the output voltage Vo2 (5V in a normal operation) after the time point t2, which violate the general specification of the power supply circuit.
FIG. 2A and FIG. 2B show another conventional power supply circuit which is designed to overcome the drawbacks in the above-mentioned prior art. As shown in FIG. 2A, the power supply circuit 10 connects a diode D1 between the output voltage Vo2 of the synchronous buck switching regulator 14 and the output voltage Vo3 of the synchronous buck switching regulator 16. The positive terminal of the diode is connected to the lower output voltage Vo3 (3.3V in a normal operation), and the negative terminal of the diode is connected to the higher output voltage Vo2 (5V in a normal operation). Hence, when the power supply circuit 10 enters the shutdown procedure, the signal waveforms of the input voltage Vin and the output voltages Vo1, Vo2 and Vo3 are as shown in FIG. 2B. That is, when the shutdown procedure begins at the time point t0, the input voltage Vin begins to decrease. When the input voltage Vin decreases to the Under-Voltage Lockout threshold UVLOth at the time point t1′, the synchronous buck switching regulators 14 and 16 stop converting the input voltage Vin to the output voltages Vo2 and Vo3, and the load circuits begin to decrease the output voltages Vo2 and Vo3. Assuming that the loading current I2 is greater than the loading current I3, between the time points t2′ and t3′, owing to the presence of the diode D1, when the output voltages Vo3 is greater than the output voltages Vo2 plus the forward voltage of the diode D1, the charges stored in the output capacitor Co3 will flow to the terminal of the output voltage Vo2 via the diode D1 and will be depleted by the loading current I2 to the output Vo2. At the time point t3′, the output voltage Vo3 decreases below the forward voltage of the diode D1; at the time point t4′, the output voltage Vo3 decreases to 0V.
Although the prior art shown in FIG. 2A overcomes the drawbacks of the prior art shown in FIG. 1A, the prior art shown in FIG. 2A still has several drawbacks as follow:                (1) The use of a diode increases the cost.        (2) Since it is desired for the diode to have a very small forward voltage, such diode having such very small forward voltage requires an even higher cost.        (3) The output voltage Vo3 is still greater than (although with a small amplitude) the output voltages Vo2 between the time points t2′ and t4′, which may lead to an undesired operation of the load circuit.        
In view of above, to overcome the drawbacks in the prior art, the present invention proposes a switching regulator which ensures the output voltages Vo1, Vo2 and Vo3 to have the relationship Vo1>Vo2>Vo3 in the shutdown procedure, regardless of the loading conditions of the load circuits. In addition, the waveforms of the output voltages generated by the proposed switching regulator are monotonic. The present invention also provides a method for controlling a switching regulator, which has the above advantages as well.